1. Field of the Invention
The present invention relates to a waveguide/planar line converter and a high frequency circuit arrangement and, more particularly, to a waveguide/planar line converter whereby the transmission mode of a high frequency signal such as a microwave or a millimeter wave can be converted, and a high frequency circuit arrangement.
2. Description of the Relevant Art
In sending-receiving sections of radar sensors of various kinds, such as a mobile radar system, a waveguide/planar line converter has been used for outputting a high frequency signal sent from a high frequency circuit to an antenna with a conversion from the planar line mode to the waveguide mode, or inputting a high frequency signal received through the antenna to the high frequency circuit with a conversion from the waveguide mode to the planar line mode.
FIG. 10 is a perspective view partly in section schematically showing a conventional waveguide/planar line converter. FIGS. 11(a) and 11(b) are schematic diagrams showing a waveguide/planar line conversion substrate, wherein FIG. 11(a) is a top view thereof, while FIG. 11(b) is a bottom view thereof.
Reference numeral 50 in FIG. 10 represents a metallic housing. At a prescribed location on the housing 50, a waveguide 50a comprising a through hole is formed. Over the mouth of the waveguide 50a, a metallic short-circuiting lid 51 is arranged. The distance between the top surface of a protrusion portion of a waveguide/planar line conversion substrate 52 and the inner surface of the short-circuiting lid 51 opposed to the above-mentioned top surface is set to be about λ/4 (where λ is a wavelength of a millimeter wave or the like within the waveguide) so that the inner surface of the short-circuiting lid 51 becomes a short-circuiting plane.
As shown in FIG. 10, on the top surface of the housing 50, the waveguide/planar line conversion substrate 52 is arranged in such a manner that one end portion thereof protrudes into the waveguide 50a. As shown in FIGS. 11(a) and 11(b), on the top surface of a dielectric substrate 52a constituting the main body of the waveguide/planar line conversion substrate 52, a signal line 52b propagating high frequency signals and a patch portion 52c located above the mouth of the waveguide 50a are formed, as shown in FIG. 11(a), while on the bottom surface thereof except the portion thereof protruding into the waveguide 50a, a ground 52d is formed, as shown in FIG. 11(b). The waveguide/planar line conversion substrate 52 comprises the dielectric substrate 52a, signal line 52b, patch portion 52c and ground 52d. 
As shown in FIG. 10, in the vicinity of the other end portion of the waveguide/planar line conversion substrate 52 on the top of the housing 50, high frequency ICs 53 are mounted, which are electrically connected to the signal line 52b. Around the high frequency ICs 53 on the top of the housing 50, an interconnection substrate 54 in which circuits of various kinds or interconnections are formed is mounted.
On the other hand, as shown in FIG. 10, on the bottom surface of the housing 50, a plane antenna 55 is arranged for receiving a high frequency signal from an outside source to output it to the waveguide 50a or for emitting externally a high frequency signal transmitted through the waveguide 50a. 
In such a waveguide/planar line converter, a high frequency signal (such as a microwave or a millimeter wave) received through the plane antenna 55 propagates within the waveguide 50a, reaches the inner surface of the short-circuiting lid 51 so as to make a short circuit. Consequently, the high-frequency electric field peaks in the vicinity of the patch portion 52c of the waveguide/planar line conversion substrate 52. Therefore, in the waveguide/planar line conversion substrate 52, as shown in FIGS. 10, 11(a) and 11(b), the high frequency signal is efficiently converted from the waveguide mode to the planar line mode, and the high frequency signal converted into the planar line mode propagates through the signal line 52b to be transmitted to the high frequency ICs 53.
On the other hand, a high frequency signal output from the high frequency ICs 53 propagates in the planar line mode through the signal line 52b of the waveguide/planar line conversion substrate 52, and with a conversion from the planar line mode to the waveguide mode in the patch portion 52c, it is emitted into the waveguide 50a to be transmitted to the plane antenna 55.
However, in the conventional waveguide/planar line converter, since the opening size of the waveguide 50a becomes much smaller (e.g. about 2.54 mm×1.27 mm for a millimeter wave of 76 GHz), the waveguide/planar line conversion substrate 52 which is arranged with a protrusion into the waveguide 50a needs to be processed in accordance with the size of the small opening. Therefore, the size of the substrate cannot be made larger, resulting in complicated substrate processing. Moreover, it is becoming more difficult to accurately fit together the mouth of the waveguide 50a and the patch portion 52c of the waveguide/planar line conversion substrate 52. Therefore, if a displacement or the like is caused, the matching characteristic is degraded, so that it becomes impossible to obtain a high conversion efficiency.
The waveguide/planar line conversion substrate 52 is required to be a double-sided substrate in order to form a microstrip line thereon, so that it is constituted as a single component. Therefore, the sharing thereof with another component such as the interconnection substrate 54 has not been achieved. As a result, the circuit area formed on the top of the housing 50 becomes large, thus preventing a significant downsizing of the device.
In the conventional waveguide/planar line converter, the metallic housing 50 is used as a platform, resulting in a high cost and a difficulty in weight reduction.